Such a line circuit wherein the direct current is produced by the amplifier is known from the U.S. Pat. No. 4,041,252.
Subscriber's exchanges operating on the basis of time-division multiplex or small signal spatial networks require a cheap subscriber's line circuit on a "one per line" basis. This requires the highest possible integration degree of the circuit.
The supply source comprises a d.c. voltage source for the supply of the direct current. The value of the direct current depends on the length of the subscriber's line. To ensure a line current of a sufficient value for very long lines, the voltage of the voltage source is normally 48 or 60 volts. Consequently, the line current for very short lines or in case of a short circuit on the subscriber's line can be very high. Telephone administrations therefore prescribe a 400 to 800 Ohm supply resistor arranged in series with the supply source, by means of which the maximum line current is limited to 100 to 120 mA. As a subscriber's line is a two-wire transmission line, the line circuit comprises a second amplifier which is connected so that the line circuit has a symmetrical construction. The supply resistor then serves as a terminating impedance for the line divided into two impedances, each having a value of 200 to 400 Ohm. To prevent these impedances from converting common mode signals on the subscriber's line into differential mode signals, the administrations require an accuracy of approximately 0.1% for these impedances. Owing to the high value of the maximum line current, the output transistors of the amplifiers dissipate a relatively high power. This dissipation in the portion to be integrated has formed so far an obstacle for the use of these circuits with the life and reliability required for telephony systems.
It is an object of the invention to mitigate this drawback and to provide a line circuit whose dissipation in the active components of the line circuit is very low so that they can be easily realised in integrated form.